Add a binary format for incoming messages

The existing, general incoming message endpoint accepts messages as
JSON strings containing base64 data, along with all the metadata as
other JSON keys. That's not very efficient, and we don't make use of
that full generality anyway. This commit introduces a new binary
format that supports everything we're using from the old format (with
the help of some query parameters like multi-recipient messages).

service/src/main/java/org/whispersystems/textsecuregcm/entities/IncomingDeviceMessage.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.entities;
+
+import javax.validation.constraints.Max;
+import javax.validation.constraints.Min;
+import javax.validation.constraints.NotNull;
+
+public class IncomingDeviceMessage {
+  private final int type;
+
+  @Min(1)
+  private final long deviceId;
+
+  @Min(0)
+  @Max(65536)
+  private final int registrationId;
+
+  @NotNull
+  private final byte[] content;
+
+  public IncomingDeviceMessage(int type, long deviceId, int registrationId, byte[] content) {
+    this.type = type;
+    this.deviceId = deviceId;
+    this.registrationId = registrationId;
+    this.content = content;
+  }
+
+  public int getType() {
+    return type;
+  }
+
+  public long getDeviceId() {
+    return deviceId;
+  }
+
+  public int getRegistrationId() {
+    return registrationId;
+  }
+
+  public byte[] getContent() {
+    return content;
+  }
+}